Orthogonal Frequency Division Multiplexing (OFDM) is a linear modulation scheme in which a set of orthogonal subcarriers are used to carry user data. OFDM can be efficiently implemented using Fourier transforms, such as the Fast Fourier Transform (FFT) and can be designed to be especially robust to multipath. Low complexity frequency domain equalization algorithms can be used to mitigate the impact of the channel. OFDM may provide a spectrally efficient scheme in which the subcarriers can overlap but remain orthogonal (in the absence of synchronization errors).
OFDMA (orthogonal frequency division multiple access) is an extension of OFDM in which multiple users share the same transmission band. OFDMA is a multi-user version of the OFDM modulation scheme. The multiple access feature is achieved in OFDMA by assigning subsets of the subcarriers to individual users, and allows the simultaneous low data rate transmission from several users over the same band. In conventional OFDMA, a different number of the available sub-carriers can be assigned to different users, in order to support differentiated quality of service (QoS).
A disadvantage of multi-carrier modulation, such as OFDM or OFDMA, is that it can exhibit high peak-to-average power ratio (PAPR), where the peak value of the signal can be much larger than the average (typical) value. For example, the peak value of an OFDM waveform may typically grow linearly with the number of subcarriers because for some OFDM symbols, all of the subchannel waveforms can potentially add up together in phase at some time during the transmission. This may require the use of circuits with linear characteristics over a large dynamic range, such as linear power amplifiers. However, most power amplifiers may he most power-efficient when they are operating in a saturation (or nonlinear) region. In general, linear power amplifiers are less efficient, more expensive, and often require larger areas (for heat dissipation) than their nonlinear counterparts. Furthermore, clipping the signal at high levels may occur which may distort the information-bearing amplitude of the signal and yields out-of-band radiation, and which may degrade bit error rate (BER) performance of the system.